Tidal Range Energy Resource and Optimization – Past

1 Tidal range energy resource and optimization – past

2 perspectives and future challenges

a,∗ b a 3 Simon P. Neill , Athanasios Angeloudis , Peter E. Robins , Ian c a d a 4 Walkington , Sophie L. Ward , Ian Masters , Matt J. Lewis , Marco a b b f 5 Piano , Alexandros Avdis , Matthew D. Piggott , George Aggidis , Paul g h f e 6 Evans , Thomas A. A. Adcock , Audrius Zidonisˇ , Reza Ahmadian , Roger e 7 Falconer

a 8 School of Ocean Sciences, Bangor University, Marine Centre Wales, Menai Bridge, UK b 9 Department of Earth Science and Engineering, Imperial College London, UK c 10 School of Natural Sciences and Psychology, Department of Geography, Liverpool John 11 Moores University, Liverpool, UK d 12 College of Engineering, Swansea University, Bay Campus, Swansea, UK e 13 School of Engineering, Cardiff University, The Parade, Cardiff, UK f 14 Engineering Department, Lancaster University, Lancaster, UK g 15 Wallingford HydroSolutions, Castle Court, 6 Cathedral Road, Cardiff, UK h 16 Department of Engineering Science, University of Oxford, Oxford, UK

17 Abstract

18 Tidal energy is one of the most predictable forms of renewable energy. Al-

19 though there has been much commercial and R&D progress in tidal stream

20 energy, tidal range is a more mature technology, with tidal range power plants

21 having a history that extends back over 50 years. With the 2017 publication

22 of the “Hendry Review” that examined the feasibility of tidal lagoon power

23 plants in the UK, it is timely to review tidal range power plants. Here, we

24 explain the main principles of tidal range power plants, and review two main

25 research areas: the present and future tidal range resource, and the opti-

26 mization of tidal range power plants. We also discuss how variability in the

27 electricity generated from tidal range power plants could be partially oﬀset

28 by the development of multiple power plants (e.g. lagoons) that are comple-

Preprint∗Corresponding submitted toauthor Renewable Energy May 2, 2018 Email address: [email protected] (Simon P. Neill) 29 mentary in phase, and by the provision of energy storage. Finally, we discuss

30 the implications of the Hendry Review, and what this means for the future

31 of tidal range power plants in the UK and internationally.

32 Keywords: Tidal lagoon, Tidal barrage, Resource assessment,

33 Optimization, Hendry Review, Swansea Bay

34 Contents

35 1 Introduction 3

36 2 A brief history of tidal range schemes 7

37 2.1 Commercial progress ...... 7

38 2.1.1 Current schemes ...... 7

39 2.1.2 Proposed schemes ...... 9

40 2.2 Engineering aspects of tidal range power plants ...... 10

41 3 Numerical simulations of tidal range power plants 12

42 3.1 0D modelling ...... 12

43 3.2 1D modelling ...... 14

44 3.3 2D and 3D models ...... 15

45 3.4 Observations and validation ...... 19

46 4 Tidal range resource 20

47 4.1 Theoretical global resource ...... 20

48 4.2 Theoretical resource of the European shelf seas ...... 22

49 4.3 Non-astronomical inﬂuences on the resource ...... 26

50 4.4 Long timescale changes in the tidal range resource ...... 27

2 51 4.5 Socio-techno-economic constraints on the theoretical resource . 28

52 5 Optimization 31

53 5.1 Energy optimization ...... 31

54 5.2 Economic optimization ...... 33

55 5.3 Implications of regional hydrodynamics for individual lagoon

56 resource ...... 34

57 5.4 Multiple lagoon resource optimization ...... 35

58 6 Challenges and opportunities 37

59 6.1 Variability and storage ...... 37

60 6.2 Additional socio-economic beneﬁts through multiple use of space 39

61 6.3 Implications of the Hendry Review ...... 40

62 7 Conclusions 42

63 1. Introduction

64 Much of the energy on Earth that is available for electricity generation,

65 particularly the formation of hydrocarbons, originates from the Sun. This

66 also includes renewable sources of electricity generation such as solar, wind

67 & wave energy, and hydropower (since weather patterns are driven, to a

68 signiﬁcant extent, by the energy input from the Sun). However, one key

69 exception is the potential for electricity generation from the tides – a result

70 of the tide generating forces that arise predominantly from the coupled Earth-

3 1 71 Moon system . The potential for converting the energy of tides into other

72 useful forms of energy has long been recognised; for example tide mills were

73 in operation in the middle ages, and may even have been in use as far back as

74 Roman times [1]. The potential for using tidal range to generate electricity

75 was originally proposed for the Severn Estuary in Victorian times [2], and

76 La Rance (Brittany) tidal barrage – the world’s ﬁrst tidal power plant – has

77 been generating electricity since 1966 [3]. However, only very recently has the

78 strategic case for tidal lagoon power plants been comprehensively assessed,

79 with the publication of the “Hendry Review” in January 2017 [4].

80 Tidal range power plants are deﬁned as dams, constructed where the

81 tidal range is suﬃcient to economically site turbines to generate electricity.

82 The plant operation is based on the principle of creating an artiﬁcial tidal

83 phase diﬀerence by impounding water, and then allowing it to ﬂow through

84 turbines. The instantaneous potential power (P ) generated is proportional

85 to the product of the impounded wetted surface area (A) and the square of

86 the water level diﬀerence (H) between the upstream and downstream sides

87 of the impoundment: P ∝ AH2 (1)

88 A tidal range power plant consists of four main components [5, 6]:

89 • Embankments form the main artiﬁcial outline of the impoundment, and

90 are designed to have a minimal length while maximizing the enclosed

91 plan surface area. A key factor in designing the embankment is to

1The Sun also has an important role in tides, but its contribution is around half that of the Moon.

4 92 minimise disturbance to the natural tidal ﬂow.

93 • Turbines are located in water passages across the embankment, and

94 convert the potential energy created by the head diﬀerence into rota-

95 tional energy, and subsequently into electricity via generators.

96 • Openings are ﬁtted with control gates, or sluice gates, to transfer ﬂows

97 at a particular time, and with minimal obstruction.

98 • Locks are incorporated along the structure to allow vessels to safely

99 pass the impoundment.

100 Tidal range power plants can be either coastally attached (such as a bar-

101 rage) or located entirely oﬀshore (such as a lagoon). The primary diﬀerence

102 between the two refers to their impoundment perimeter. There are also

103 coastally-attached lagoons, where the majority of the perimeter is artiﬁcial,

104 potentially enabling smaller developments with more limited environmental

105 impacts than barrages – the latter generally spanning the entire width of an

106 estuary.

107 Following construction, the manner and how much of the potential energy

108 is extracted from the tides largely depends on the regulation of the turbines

109 and sluice gates [7]. They can be designed to generate power one-way, i.e.

110 ebb-only or ﬂood-only, or bi-directionally. In one-way ebb generation, the

111 rising tide enters the enclosed basin through sluice gates and idling turbines.

112 Once the maximum level in the lagoon is achieved, these gates are closed,

113 until a suﬃcient head (hmax) develops on the falling tide. Power is subse-

114 quently generated until a predetermined minimum head diﬀerence (hmin),

115 when turbines are no longer operating eﬃciently. For ﬂood generation the

5 116 whole process is reversed to generate power during the rising tide. In two-way

117 power generation, energy is extracted on both the ﬂood and ebb phases of the

118 tidal cycle, with sluicing occurring around the times of high and low water

119 [8, 9]. A schematic representation of ebb and two-way generation modes of

120 operation is shown in Fig. 1, highlighting the main trigger points during the

121 tidal cycle that dictate power generation. Nonetheless, there are other pos-

122 sible variations of these regimes (e.g. Section 5.1). For example, ebb/ﬂood

123 generation can often be supplemented with pumping water through the tur-

124 bines to further increase the water head diﬀerence values, as considered in

125 studies by Aggidis and Benzon [10] and Yates et al. [11].

126 In this article, we provide a review of tidal range power plants, with a fo-

127 cus on resource and optimization. The following section provides an overview

128 of the history of tidal range schemes from pre-industrialization to present day,

129 including future proposed schemes. Section 3 compares the various modelling

130 approaches used to simulate tidal lagoon or barrage operation (e.g. 0D versus

131 2D models), and Section 4 examines the global tidal range resource, with a

132 particular focus on the northwest European continental shelf, and constraints

133 on the development of this resource. Section 5 examines ways in which tidal

134 range schemes can be optimised, e.g. ﬂood or ebb generation, pumping, and

135 the beneﬁts of concurrently developing multiple tidal range schemes. Finally,

136 in Section 6, we discuss future challenges and opportunities facing tidal range

137 power plants, including variability and storage, and the implications of the

138 Hendry Review.

6 139 2. A brief history of tidal range schemes

140 Tidal range technologies have a long history, especially when compared

141 with less mature ocean energy technologies such as tidal stream and wave

142 energy. Energy has been extracted from the tides for centuries. There is

143 evidence of a tide mill in Strangford Lough, Northern Ireland, which has been

2 144 dated to the early 6th Century [1], where an 8 m wide dam enclosed a 6500 m

145 area of sea water. Such early tidal power plants worked much as modern tidal

146 range projects, but used only naturally-occurring tidal basins to impound

147 volumes of water, which would then be routed through a paddlewheel or

148 waterwheel during the ebb. The extracted energy was, of course, not used to

149 generate electricity, but to provide mechanical motion, for example to mill

150 grain.

151 2.1. Commercial progress

152 Locations around the world that are suitable for tidal range exploitation

153 are relatively limited, given a number of physical constraints, including tidal

154 range, grid connectivity, geomorphology, seabed conditions, and available

155 area for an impoundment. There are ﬁve tidal range power plants currently

156 in operation around the world, and a number of areas that have either been

157 identiﬁed for development, or which exhibit suitable characteristics to merit

158 consideration.

159 2.1.1. Current schemes

160 La Rance tidal barrage in Brittany was the world’s ﬁrst fully operational

161 tidal power station [3, 12, 13]. The project, which comprises a 720 m long

2 162 barrage and impounds an area of approximately 22 km [14], was constructed

7 163 over a six-year period, and was fully operational in 1966 (Table 1). The

164 barrage houses 24 Kaplan bulb turbines, which provide a combined rated

165 power output of 240 MW and an annual energy output of 480 GWh [15].

166 Since its inception, there have not been any major structural issues, and very

167 little downtime, although there have been signiﬁcant environmental impacts

168 [16].

169 The Kislaya Guba tidal power plant in Russia was constructed in 1968

170 as a trial project by the government, with an initial installed capacity of 400

171 kW [14]. It is situated near Murmansk, a fjord on the Kola Peninsula [13].

172 The installed capacity of this power plant has grown to 1.7 MW, which is

173 relatively low compared with other worldwide schemes, making it the smallest

174 tidal range power plant in operation [17]. However, the success of this scheme

175 has motivated the government to explore other sites, including Mezan Bay in

176 the White Sea and Tugar Bay, with potential installed capacities of 15 GW

177 and 6.8 GW respectively [17]. The former of the two ﬁgures is particularly

178 impressive, since this would be the second largest power plant in the world,

179 the largest being the 22.5 GW Three Gorges Dam in China [18].

180 The Annapolis Royal Generating Station was constructed in 1984, and

181 is located on the Annapolis River, Nova Scotia, Canada. It harnesses the

182 head diﬀerence created in the Annapolis Basin, a sub-basin of the Bay of

183 Fundy, which has a spring tidal range of 16 m [19]. This scheme consists

184 of a single Straﬂo turbine, and produces a peak power output of 20 MW on

185 the ebb tide only [13]. As well as generating electricity, this power plant is

186 also used for ﬂood defence and serves as an important transport link – the

187 latter being a particularly advantageous and unique feature of barrages, for

8 188 example compared to a tidal lagoon.

189 The Jiangxia tidal range power plant was opened in 1985, and is located in

190 Jiangxia Port, Wenling, China, an area that is characterised by tidal ranges

191 of up to 8.4 m [13]. The power plant operates bi-directionally, and houses six

192 bulb turbines, the last of which was installed in 2007, providing an installed

193 capacity of 3.9 MW.

194 The largest (by installed capacity) tidal range scheme currently in exis-

195 tence is Lake Sihwa, which is situated in the mid-eastern region of the Korean

196 Peninsula in the Kyeonggi Bay, South Korea. The power plant stemmed from

197 a disused dam constructed in 1994 to hold irrigation water for agricultural

198 land; however, industrial developments in its vicinity caused pollution issues

199 [20]. To help tackle the pollution problems, the dam was subsequently con-

200 verted to a ﬂood-operating tidal power plant [13]. The power plant incorpo-

201 rates 10 bulb turbines, with an installed capacity of 254 MW. The success of

202 this scheme has motivated the Korean government to explore other potential

203 sites around the country, including Gerolim and Incheon [13].

204 2.1.2. Proposed schemes

205 There are a number of factors that preclude development in certain areas,

206 even if ﬁrst-order theoretical appraisals of the resource suggest that there is

207 commercial potential. Apart from physical constraints, cost and environmen-

208 tal impacts are other major barriers to development. Environmental issues,

209 particularly for larger scale schemes, have prevented numerous developments

210 from being approved [13]. Without constructing a scheme, its true environ-

211 mental impact is diﬃcult to quantify, and so governments are hesitant to

212 proceed with development at such scale. Table 2 summarises sites around

9 213 the world that have the potential for tidal range exploitation.

214 A relatively recent tidal range concept that addresses some of these envi-

215 ronmental concerns is the tidal lagoon. These tidal range power plants diﬀer

216 from the more conventional barrage schemes, as they impound a smaller body

217 of water and are therefore less intrusive. One such scheme is the proposed

218 Swansea Bay Lagoon, located in the Bristol Channel, UK, an area that is

219 characterized by tidal ranges that exceed 10 m [21].

220 Although no tidal lagoons currently exist, the Swansea Bay Lagoon is

221 the closest scheme to commercial viability. The UK Government have re-

222 cently completed an independent review which considered the feasibility of

223 the power plant in terms of cost eﬀectiveness, supply chain opportunities,

224 possible structures to ﬁnance this project, and scales of design [22]. Despite

225 the positive outcome of the “Hendry Review” [4], a marine licence is still re-

2 226 quired from Natural Resources Wales (NRW) , and an agreement on the CfD

227 (Contracts for Diﬀerence) price, before the project can proceed to construc-

228 tion. There are a number of other areas in the UK that have been identiﬁed

229 for development, as summarized in Table 2. However, it is likely that these

230 will only be approved on the condition that the Swansea Bay “Pathﬁnder

231 Project” proceeds and is successful.

232 2.2. Engineering aspects of tidal range power plants

233 Bulb turbines are used for power takeoﬀ in almost all current tidal range

234 schemes [13]. These are the same, or very similar, to the turbines that are

235 used for low head hydropower applications. When low head hydro was con-

2NRW is an environmental body sponsored by the Welsh Government.

10 236 sidered as an energy solution for the UK in 1927, the investigating team (the

237 Severn Barrage Committee) found the Kaplan turbine to be the most eﬃ-

238 cient for low head applications [23]. In the following years, as more research

239 has been conducted in the ﬁeld of turbines, the bulb turbine, a conﬁguration

240 of a Kaplan turbine, has become the turbine of choice for low head hydro or

241 tidal range schemes. Furthermore, triple regulation (adjustable guide vanes,

242 blade pitch angle and variable speed) of turbines has become feasible in re-

243 cent years [4, 13, 21], which will accommodate the constant varying head

244 conditions that are inevitable in tidal range applications.

245 Tidal range schemes will likely utilise this relatively mature turbine tech-

246 nology, with speciﬁc adaptations to better suit tidal environments. It is most

247 certain that the largest share of the cost is in the civil engineering work [4].

248 A potential reduction of the civil costs is proposed, which is the usage of

249 caissons. This would enable the construction of the turbine housing struc-

250 ture on land, as opposed to using coﬀerdams. It has to be taken into account

251 that in tidal range applications a longer water passage is required, as the

252 bulb turbines may work in two-way generation, as opposed to classical one-

253 way generation [7, 24]. Therefore, a draft tube is required on both sides of

254 the turbine. Recent suggestions for impoundment designs include the use of

255 geotubes and sand [6, 13]. These impoundments would also act as break-

256 water and sea defence structures, helping protect neighbouring regions from

257 ﬂooding [e.g. 25].

11 258 3. Numerical simulations of tidal range power plants

259 The assessment of tidal range schemes relies on the development of nu-

260 merical tools that can simulate their operation over time. These span from

261 simpliﬁed theoretical and zero-dimensional (0D) models [8, 10, 26, 27] to more

262 sophisticated depth-averaged (2D) and hydro-environmental tools [9, 20, 24,

263 28, 29, 30, 31, 32, 33, 34, 35, 36, 37] that often require High Performance

264 Computing (HPC) capabilities for practical application.

265 3.1. 0D modelling

266 Given (a) known tidal conditions, (b) plant operation sequence, and (c)

267 appropriate formulae that represent the performance of constituent hydraulic

268 structures, it is feasible to simulate the overall performance of a tidal range

269 scheme, and provide an informed resource assessment [24]. The operation can

270 be modelled using a water level time series as input, governed by the transient

271 downstream water elevations at the site location (Fig. 1). This is known as

272 0D modelling, and has been deemed suﬃcient under certain conditions, e.g.

273 for smaller lagoons and barrages, as explored in the literature [28, 34, 35, 38].

274 A multitude of 0D models have been reported for the estimation of tidal

275 power plant electricity outputs [e.g. 27, 34, 39]. However, one commonly used

276 technique is the backward-diﬀerence numerical model, developed according

277 to the continuity equation. Given the downstream ηdn,i and upstream ηup,i

278 water level at any point in time t (indicated by subscript i), the upstream

279 water level at t + δt (subscript i + 1) can be calculated as [27]:

Q(Hi) + Qin,i ηup,i+1 = ηup,i + ∆t (2) A(ηup,i)

12 280 where A(ηup) is the wetted surface area of the lagoon, assuming a constant

281 water level surface of ηup. Qin corresponds to the sum of inﬂows/outﬂows

282 through sources other than the impoundment, e.g. rivers or outﬂows. The

283 water head diﬀerence H is deﬁned as ηup,i − ηdn,i, and feeds into Q(H); a

284 function for the total discharge contributions from turbines and sluice gates.

285 Theoretically, the ﬂow through a hydraulic structure is calculated as [5]: p Q = CDAs 2gH (3)

286 where CD is a discharge coeﬃcient, and As is the cross-sectional ﬂow area.

287 In turn, the power P produced from a tidal range turbine for a given H can

288 be:

P = ρgQT Hα (4)

289 where ρ is the ﬂuid density, QT is the turbine ﬂow rate and α is an over-

290 all eﬃciency factor associated with the turbines. In practice, the hydraulic

291 structure ﬂow rates and power output should be represented by hill charts

292 speciﬁc to the individual characteristics of sluice gates and turbines, thus

293 incorporating their technical constraints. Examples of such charts for bulb

294 turbine designs can be found in the literature [e.g. 40, 41].

295 The ﬂow rate Q and power P are also subject to the operation mode of

296 the plant (Fig. 1), which will accordingly restrict/allow ﬂow through turbines

297 and sluice gates at certain times within the tidal cycle. Details of one-way

298 and two-way generation algorithms that dictate the modes of operation over

299 time have been presented in Angeloudis and Falconer [24], with variations

300 schematically represented in several studies [e.g. 28, 30, 34, 35].

301 Even though a 0D modelling approach is computationally eﬃcient, it of-

302 ten assumes that the impact of the tidal impoundment itself on the localised

13 303 tidal levels is negligible. Such an assumption can yield over-optimistic re-

304 sults, as reported in Angeloudis and Falconer [27] and Yates et al. [11].

305 Consequently, the analysis should be expanded to account for the regional

306 hydrodynamic impacts through reﬁned coastal modelling tools tailored to

307 the operation of tidal lagoons.

308 3.2. 1D modelling

309 Many candidate sites for tidal range schemes are on estuaries, where it

310 is possible to integrate the ﬂow both vertically and across the width of the

311 estuary [e.g. 42]. Such models may be useful for modelling tidal lagoons and

312 barrages, as they are able to capture some of the changes to tidal hydro-

313 dynamics due to the presence and operation of the tidal range power plant

314 [38] without the computational demands of more complex models. There are

315 numerous examples of 1D modelling being used to simulate tidal barrages;

316 examples include semi-analytical models [43, 44, 45] and numerical modelling

317 [39, 46, 47, 48]. Upstream and downstream sections of a tidal range scheme

318 can be simulated independently as two coupled 1D models. For a barrage

319 scheme, the constituent sections are linked at the respective ends, whereas

320 tidal lagoons are treated as junctions to the main channel section [49].

321 However, conclusions drawn from 1D models need to be treated with

322 caution. Due to the simpliﬁcations inherent in a 1D model, the naturally

323 occurring amplitude (i.e. without the barrage present) at the barrage loca-

324 tion may be poorly represented (in comparison to 2D models). In general, it

325 has been demonstrated that the performance of 1D models is adequate for

326 simulating relatively small tidal projects (e.g. the Swansea Bay lagoon), but

327 insuﬃcient for simulating larger schemes such as a large barrage [49]. There-

14 328 fore, signiﬁcant error bars should be placed on the output from such models.

329 Nevertheless, 1D models are useful qualitatively for assessing the scale of the

330 impact of placing barrages in estuaries, and also useful for analysing operat-

331 ing strategies where computationally eﬃcient models are required to explore

332 or optimise multiple scenarios.

333 3.3. 2D and 3D models

334 Hydrodynamic simulations of coastal waters can provide valuable insight

335 into resource assessment, the quantiﬁcation of the potential impacts from

336 planned coastal engineering projects, and the minimization of any detri-

337 mental eﬀects through design optimization. In principle, the capability of

338 depth-averaged (2D) and three-dimensional (3D) numerical models to pro-

339 duce time-series approximations to primitive variable ﬁelds, such as velocity

340 and free-surface elevation, make them attractive tools for the study of the

341 extractable energy and potential impacts of coastal engineering structures.

342 However, a wide range of multi-scale processes must be either directly simu-

343 lated or parameterized in order to ensure the appropriate levels of accuracy

344 required to make them useful tools for impact assessment and optimization

345 studies within planning, operational and research contexts. In particular,

346 tidal, ﬂuvial and wave dynamics, as well as biogeochemical and sedimen-

347 tological processes, can be considered in both the near- and far-ﬁelds. In

348 addition, engineering structures such as turbines, sluices and impoundments

349 need to be incorporated. A formally complete and accurate representation

350 (e.g. via direct numerical simulation) of all these processes is beyond present

351 computational capabilities. As a result, various approximations are employed

352 to study aspects of hydrodynamic ﬂows and environmental impacts. The dif-

15 353 fering levels of approximation used to model impoundments are outlined in

354 this section, ordered in terms of dimensionality of the solution space.

355 For the majority of research to-date, especially at larger regional scales,

356 the depth-averaged (2D) shallow water equations (SWE) have been adapted

357 to assess the potential resource and impacts of tidal range schemes. These

358 are obtained following the depth-integration of the Navier-Stokes equations

359 which govern ﬂuid ﬂow in 3D, under the assumptions that horizontal length

360 scales are much greater than vertical scales, and pressure is close to being

361 in hydrostatic balance. It is common for these equations to be considered in

362 both non-conservative, as well as the following conservative forms: ∂U ∂E ∂G ∂E˜ ∂G˜ + + = + + S (5) ∂t ∂x ∂y ∂x ∂y

363 where U is the vector of conserved variables, E and G are the convective ﬂux

364 vectors in the x and y direction respectively, E˜ and G˜ are diﬀusive vectors

365 in the x and y directions, and S is a source term that includes the eﬀects of

366 bed friction, bed slope and the Coriolis force. The terms in Eq. 5 can be

367 expanded as [30]:

        h hu hv 0    1         2 2   ˜   U = hu ,E = hu + gh  ,G =  huv  , E = τxx ,...    2      2 1 2 hv huv hv + gh τxy 2 (6)

    0 qs     ˜     ... G = τxy ,S = +hfv + gh(Sbx − Sfx)     τyy −hfu + gh(Sby − Sfy)

16 368 where u, v are the depth-averaged horizontal velocities in the x and y direc-

369 tion, respectively, h is the total water depth, and qs is the source discharge

370 per unit area. The variables τxx, τxy, τyx and τyy represent components of

371 the turbulent shear stresses over the plane, and f refers to the Coriolis ac-

372 celeration. Here the bed and friction slopes have been denoted for the x and

373 y directions as Sbx, Sby and Sfx, Sfy respectively.

374 For coastal ocean models, when solving either the 2D SWE or the hydro-

375 static or non-hydrostatic forms of the 3D Navier-Stokes equations, the ﬁrst

376 decision generally made is whether the domain in question can be adequately

377 described at a discrete level using a structured mesh, or if the ﬂexibility af-

378 forded by an unstructured mesh is desired. The latter is particularly useful

379 when accurate representation of complex geometries is required, and/or dras-

380 tically diﬀerent spatial mesh resolution is desired within a single computa-

381 tional domain [50]. A key decision is then often whether open source versus

382 proprietary software is used, and in the case of unstructured meshes whether

383 a ﬁnite volume or ﬁnite element based discretization approach is employed.

384 For the solution of the governing equations, previous studies have applied

385 a variety of coastal models including ADCIRC [35], Telemac-2D [9], EFDC

386 [32, 51], as well as in-house research-focused software [24, 30].

387 A common aspect in all of these approaches is the manner in which water

388 bodies either side of the impoundment are linked numerically, given that at

389 diﬀerent times of the lagoon operation they may be completely disconnected,

390 and at others linked through sluices and turbines. A domain decomposition

391 based technique has been the standard approach employed to simulate tidal

392 lagoon operation at a ﬁeld-scale state [24, 29, 30, 32, 33, 37, 46, 51, 52].

17 393 This technique is implemented using two (or more in the case of multiple

394 impoundments) sub-domains: one upstream, and another downstream of the

395 impoundment. Open boundaries connecting the sub-domains are speciﬁed

396 in the region of ﬂow control structures, i.e. turbines and sluice gates. Sub-

397 domains are then dynamically linked using available information regarding

398 the behaviour of hydraulic structures, such as tidal turbine hill charts as with

399 simpliﬁed 0D approaches (Section 3.1). Dedicated details for the represen-

400 tation of tidal lagoons in a SWE model and the conservation of mass and

401 momentum through hydraulic structures are expanded in Angeloudis et al.

402 [52].

403 Three-dimensional studies generally commence with an extension of the

404 2D approach to include a number of vertical layers which, while having been

405 applied to other coastal engineering applications, are yet to be applied to

406 the regional scale modelling of tidal range structures. An expansion to 3D

407 layered methods would produce an appreciation of the three-dimensional con-

408 ditions generated by the hydraulic structure-induced water jets. In turn, and

409 subject to the substantial growth in the required computational resources,

410 classical 3D hydrodynamic CFD (computational ﬂuid dynamics) approaches

411 could yield even greater insight. At present, these are only generally ap-

412 plicable for smaller scale hydraulic engineering applications, due to current

413 limitations of computational resources, including storage. The use of multi-

414 scale unstructured meshes can of course blur this distinction, but one needs

415 to keep in mind the variations in time scales and the need to parameterise

416 diﬀerent turbulent processes. In fact, the expansion to fully 3D modelling of

417 tidal barrage/lagoon operations has been scarcely reported to date. At the

18 418 time of writing, this has been limited to the CFD modelling of laboratory-

419 scale ﬂows expected downstream and upstream of barrages [e.g. 53, 54, 55].

420 However, 2D models are generally accurate for predicting water levels, and

421 so for most applications, particularly resource assessments, the complexity

422 oﬀered by a 3D model is often not required.

423 3.4. Observations and validation

424 The main types of data used to parameterize and force numerical models

425 are bathymetry and boundary conditions. There are many online sources of

426 bathymetry that are suitable for model setup such as GEBCO (global 1/2

427 arc-minute grid) and EMODnet (European 1/8 arc-minute grid). However,

428 in many circumstances it may be necessary to complement such datasets

429 with local accurate high-resolution survey data, such as LiDAR or multi-

430 beam data, particularly in the inter-tidal. Although many tide gauges exist

431 around the world, providing accurate time series of water surface elevations

432 over many decades, often such datasets do not coincide with model bound-

433 aries, or are unsuitable for boundary forcing (e.g. if there are large changes

434 in amplitude and phase along a 2D boundary). Under such circumstances,

435 global or regional tidal atlases are therefore used to generate boundary condi-

436 tions. One such resource, FES2014 [56], provides both amplitude and phase

437 of surface elevations and tidal currents for 32 tidal constituents at a (global)

◦ 438 grid resolution of 1/16 × 1/16 .

439 Although it is not possible to validate a model of a lagoon prior to con-

440 struction, it is possible to validate a hydrodynamic model in the absence of

441 a lagoon. Conﬁdence in the hydrodynamic model, along with subsequent

442 rigorous parameterization of the tidal lagoon, therefore provides a tool that

19 443 can be used to explore various tidal range schemes and operating scenarios

444 prior to substantial ﬁnancial investment.

445 Generally, a thorough understanding of the resource requires that a time

446 series of the free surface is analysed and split into its astronomical compo-

447 nents (e.g. principal semi-diurnal lunar (M2) and solar (S2) constituents),

448 and it is the amplitude and phase of these constituents that forms the ba-

449 sis of model validation. However, in many circumstances, for example for

450 regions or time periods that experience signiﬁcant non-astronomical eﬀects

451 (e.g. surges), the actual time series can be used to assess the skill and accu-

452 racy of the numerical simulation.

453 4. Tidal range resource

454 4.1. Theoretical global resource

455 The analysis described below estimates the global annual theoretical tidal

456 range resource to be around 25,880 TWh, based on reasonable thresholds for

457 energy output and water depth. However, the resource is conﬁned to a few

458 coastal regions (covering 0.22% of the World’s oceans). In fact, the majority

459 of the resource is distributed across eleven countries.

460 Our global resource characterization is based solely on annual sea surface

461 elevations and water depths. The FES2014 tidal dataset was used, which

◦ ◦ 462 provides tidal elevations (amplitude and phase) at a consistent 1/16 ×1/16

463 global resolution. FES2014 is the latest iteration of the FES (Finite Element

464 Solution) tidal model, and is a considerable improvement on FES2012, par-

465 ticularly in coastal and shelf regions. Water depths were provided by the

466 GEBCO-2014 gridded bathymetry dataset (www.gebco.net), available on a

20 ◦ ◦ ◦ ◦ 467 1/120 × 1/120 global grid (which was resampled here to a 1/16 × 1/16

468 grid to match the FES2014 grid points), and referenced to mean sea level.

◦ ◦ 469 For each 1/16 × 1/16 grid cell, an annual elevation time series was con-

470 structed (using T TIDE; [57]), based on the following 5 tidal constituents:

471 M2, S2, N2, K1, and O1. For each time series, the tidal range (H) of consec-

472 utive rising and falling tides were calculated, allowing the annual potential

2 473 energy (PE, per m ), to be calculated as follows:

n X 1 PE = ρgH2 (7) 2 i i=1

474 where the subscript i denotes each successive rising and falling tide in a year

475 (n ≈ 1411), ρ is the density of seawater, and g is acceleration due to gravity.

2 476 The resulting contour map of global potential energy density (in kWh/m )

477 is shown in Fig. 2.

478 Some assumptions have been made about areas that are suitable for la-

479 goon developments, and we have calculated how much energy there is in just

480 these areas. The true limit of any development will be when the energy

481 yield does not increase the ﬁnancial return suﬃciently compared with the

482 development and running costs (Section 5.2). Here, we assume a minimum

2 483 acceptable annual energy yield of 50 kWh/m (based on the energy yield

484 from a constant tidal range of 5 m), and also a maximum water depth of 30

485 m (since construction costs of the embankment would likely be prohibitive in

486 deeper waters). Applying these criteria, the global annual potential energy is

487 approximately 25,880 TWh; distributed across the coastal regions of eleven

488 countries, as detailed in Table 3.

489 However, for the majority of the year, the largest theoretical resource,

21 490 the Hudson Bay area, contains substantial sea ice (http://nsidc.org/) and

491 steep bathymetric gradients (i.e., the resource in water depths less than 30 m

492 is constrained to the near coastal strip); and would therefore be impractical

493 to exploit. This region is also rather isolated from a demand perspective.

494 Sea ice is also prevalent in Alaska [58] and northern Russia [59], where we

495 calculated signiﬁcant potential energy. However, lagoons can be designed to

496 take account of static and dynamic ice loads on the structures. Taking into

497 account the impracticality of Hudson Bay for tidal range energy exploitation,

498 the global annual potential energy is approximately 5,792 TWh. Generally,

499 regions with desirable characteristics, i.e. regions where the tidal wave is

500 ampliﬁed due to resonance, are limited, and indeed 90% of this resource is

501 distributed across the coastal regions of just ﬁve countries, as shown in Table

502 3: Australia, Canada, UK, France, and the US (Alaska).

503 4.2. Theoretical resource of the European shelf seas

504 For more detailed analysis, we focus on the resource of the northwest

505 European shelf seas (NWESS), since this is a region that includes existing

506 (La Rance) and proposed (Swansea Bay) tidal range schemes (Section 2),

507 in addition to hosting around a quarter of the global theoretical resource

508 (Table 3). In order to estimate the NWESS tidal range resource, the 3D

509 ROMS model (Regional Ocean Modeling System) was used to simulate tidal

510 elevations, and subsequently the potential energy in both the ﬂood and ebb

◦ ◦ 511 phases of the tidal cycle. The model domain extends from 14 W to 11 E,

◦ ◦ 512 and 42 N to 62 N, but the region analysed is shown in Fig. 3. The domain

513 was discretised in the horizontal using a curvilinear grid, applying a variable

◦ 514 longitudinal resolution of 1/60 (0.87-1.38 km), and a ﬁxed latitudinal resolu-

22 ◦ 515 tion of 1/100 (∼1.11 km). The bathymetric grid is based on GEBCO global

◦ 516 data (www.gebco.net) at 1/120 resolution. The vertical model grid consists

517 of 10 layers distributed according to the ROMS terrain-following coordinate

518 system. The open boundaries of the model were forced by tidal elevation

519 (Chapman boundary condition) and tidal velocities (Flather boundary con-

520 dition), generated by 10 tidal constituents (M2, S2, N2, K2, K1, O1, P1, Q1,

◦ 521 Mf, and Mm) obtained from TPX07 global tide dataset at 1/4 resolution

522 [60]. The validation procedure for elevations, based on harmonic analysis

523 performed at 20 tide gauges distributed throughout the domain, produced

3 524 scatter indices (SI) of <8% and <6% for M2 and S2 amplitudes, respec-

525 tively. Further information about the model set up and validation can be

526 found in Robins et al. [61]. Tidal analysis from a 30-day simulation was

527 used to calculate the following 5 dominant tidal constituents, which were

528 used to construct annual elevation time series at each model grid cell: M2,

529 S2, N2, K1, and O1. Following the method outlined in Section 4.1 and using

2 530 Eq. 7, the annual energy yield (in kWh/m ) over the northwest European

531 shelf was calculated (Fig. 3).

532 Here, we assume a range of minimally acceptable annual energy yields

533 and also a maximum water depth of 30 m. Based on Tidal Lagoon Power’s

2 534 planned scheme in Swansea Bay, the lagoon has a surface area of 11.7 km

2 4 535 and a PE of approximately 84 kWh/m (i.e. a total PE of around 1 TWh) .

2 536 Other lagoon schemes typically have an annual yield of 60 kWh/m , and

537 the energy yield based on an M2 amplitude of 2.5 m is approximately 50

3Scatter Index is the RMSE normalised by the mean of the observations. 4Assuming the surface area at high tide does not reduce through the tidal cycle.

23 2 538 kWh/m .

539 If we assume initially that exploitable areas are those with water depths

2 540 <30 m and an annual yield above 50 kWh/m , then approximately 31,415

2 541 km of sea space (landward of the black contour lines in Fig. 3) is exploitable

542 throughout the NWESS, which equates to a total potential energy of 1,261

543 TWh per annum; 683 TWh per annum (54%) of which is found in UK wa-

544 ters, with the remaining 578 TWh per annum (46%) found in French waters.

545 These estimates are similar to those calculated from the global analysis (Sec-

546 tion 4.1), although the more detailed analysis here produces a 14% lower

547 resource than the global estimate, due to the improved model resolution. To

548 put these values into context, annual demand for electricity is around 309

549 TWh in the UK, and the UK theoretical tidal range resource is about double

550 this.

2 551 By increasing the threshold to 60 kWh/m , the exploitable sea space

2 552 reduces by 18% (to 26,682 km ; areas landward of the red contour lines in Fig.

553 3), but the resource decreases only slightly to 1,154 TWh per annum; 53% of

554 which is found in UK waters, with the remaining 47% found in French waters.

2 555 Increasing the threshold yield further to 84 kWh/m (the PE of Swansea Bay

556 lagoon) reduces the total resource to 832 TWh per annum (now with 44%,

557 i.e. 366 TWh, found in UK waters). Based on our criteria, the theoretical

558 resource is concentrated along the UK coasts of Liverpool Bay, the Severn

559 Estuary & Bristol Channel, the Wash, and southeast England. In France,

560 the resource is located along the northern coasts of Brittany and Normandy

561 (Fig. 3).

562 To put the above resource estimates into further context, the total M2

24 563 energy ﬂux onto the European shelf has been estimated using models and

564 satellite altimetry to be approximately 250 GW [62, 63], which equates to an

565 annual energy yield of 2,190 TWh. However, the total potential energy might

566 be higher than this, because the potential energy is moving around the system

567 all the time and, hence, it is diﬃcult to obtain a deﬁnitive theoretical value.

568 If we take energy out of the system via lagoons, it is presently unclear how

569 this will aﬀect the energy dissipation on the shelf and the energy ﬂux across

570 the shelf edge (i.e. inﬂuencing other energy systems globally). Further, since

571 discrete lagoons within the European shelf may interact with one another,

572 it is possible that the theoretical resource would alter from that calculated

573 above (Section 5.4).

574 Our resource estimates are based on theoretical energy yields, which are a

575 function of tidal range and water depths. In practice, the technical resource

576 will be considerably lower than the above theoretical estimates. For example,

577 Prandle [8] estimated that approximately 37% of the theoretical resource was

578 available for dual (ﬂood and ebb) schemes.

579 Of course, not all areas with suﬃcient yield can be exploited, due to prac-

580 tical diﬃculties with development at this scale, together with political and

581 practical constraints regarding planning. It is also unlikely that, in the near

582 future, lagoon designs would consider water depths greater than approxi-

583 mately 20 m (Mike Case, Tidal Lagoon Power; Pers. Comm.), although bar-

584 rage designs might. Therefore, our resource calculations in regions suitable

585 for lagoons should be considered an over-estimate. Moreover, it is unlikely

586 that lagoon designs at this scale could maintain the high tidal ampliﬁcation

587 near to shore. For instance, if a very large lagoon was developed, then the

25 588 tidal range within the lagoon would be reduced to approximately that at the

589 lagoon wall. Using models, lagoon optimization studies may reveal that sev-

590 eral smaller strategically sited lagoons within a region could lead to a greater

591 energy yield than one larger lagoon.

592 4.3. Non-astronomical inﬂuences on the resource

593 The previous analysis, and indeed most studies of tidal range resource,

594 assume only astronomical tides, and typically apply harmonic tide theory

595 to predict water levels. However, the tidal resource can be inﬂuenced by

596 non-astronomical eﬀects, namely storm surge. Hence, potential reliability

597 problems within tidal range energy schemes could be due to storm surges

598 [64], as negative surge events reduce the tidal range, with the converse oc-

599 curring during positive surge events. Tide-surge interaction, which results

600 in positive storm surges being more likely to occur on a ﬂooding tide [65],

601 may also reduce the annual tidal range energy resource estimate. In a recent

602 paper by Lewis et al. [64], water-level data at nine UK tide gauges suitable

603 for tidal-range energy development (i.e. where the mean tidal amplitude ex-

604 ceeds 2.5 m [23]) were used to predict tidal range power with a 0D model.

605 Storm surge aﬀected the annual resource estimate by between -5% to +3%,

606 due to inter-annual variability in the 12 year tide gauge records. However, in-

607 stantaneous power output was signiﬁcantly aﬀected (Normalised Root Mean

608 Squared Error: 3−8%, Scatter Index: 15−41%) [64]. Therefore, a prediction

609 system [e.g. 66, 67] may be required for any future electricity generation sce-

610 nario that includes a high penetration of tidal-range energy; however, annual

611 resource estimation from astronomical tides alone appears suﬃcient for re-

612 source estimation, because uncertainties in resource assessment due to design

26 613 and modelling assumptions appears greater.

614 4.4. Long timescale changes in the tidal range resource

615 Mean sea-level rise, which occurs incrementally over decadal timescales,

616 results from variations in ocean mass and ocean water density (thermosteric

617 and halosteric changes) caused by global warming and subsequent ice melt,

618 due to changes in anthropogenic or natural land-water storage and from

619 changes in ocean circulation [68]. Global mean sea level is likely to rise by

620 0.44 − 0.74 m (above the 1986 − 2005 average) by 2100 [69]. However, there

621 remain large model uncertainties in sea-level rise projections, in particular

622 when predicting the volume contribution from melting ice sheets [69], and

623 projections could increase to 1.9 m [70].

624 Future mean sea-level rise is likely to aﬀect tidal dynamics by impact-

625 ing on the position of amphidromic points and by changing resonant eﬀects

626 on shelf seas [71, 72, 73, 74], with variation in regional (relative) sea-level

627 changes due to ongoing local and far-ﬁeld isostatic eﬀects [69, 75]. In the

628 UK, observed MSL rise is broadly consistent with global MSL rise [76]. A

629 study by Ward et al. [72] indicated that projected sea-level rise over the

630 21st century is likely to alter both tidal amplitudes and tidal phases. Such

631 changes in sea levels will inﬂuence the tidal range resource, although uncer-

632 tainties in modelling the potential impacts are signiﬁcant. A preliminary

633 study by Robins et al. [77] investigated how these changes are likely to aﬀect

634 the theoretical resource at the top eight tidal range sites around the UK.

635 There was generally an increase in tidal range at these sites (1 − 3%, re-

636 sults not shown), causing the resource capacity to increase. However, when

637 the aggregated power density from multiple potential lagoon locations was

27 638 considered, tidal phase shifts tended to reduce the base-load capacity of the

639 aggregated system. In one example future scenario, simulated sea-level rise

640 clearly predicted an increased aggregated resource capacity, although the cor-

641 responding phase shifts led to reduced resource minima, which is a potential

642 consideration for ﬁrm power generation. This preliminary work can be im-

643 proved upon by considering how the feedbacks of a tidal energy extraction

644 site on the local tidal dynamics (i.e. on the resource itself) might vary with

645 changing sea levels [e.g. 72, 73].

646 4.5. Socio-techno-economic constraints on the theoretical resource

647 It is clear that not all potential tidal range sites will be developed to

648 their fullest extent. Large infrastructure projects of this type will always be

649 modiﬁed in societies where there is a democratic involvement in the planning

650 process by the local population. For example, a factor in the lack of progress

651 of the Severn barrage has been the concern of decision makers about the pub-

652 lic acceptability of the scheme. An important element of public acceptability

653 is the impact of a scheme on the local environment. This is part of planning

654 law in many countries, and within the EU is legislated by the overarching

655 Marine Strategy Framework Directive (MSFD) [78]. The most recent formal

656 review of the Severn Barrage examined environmental concerns, and con-

657 cluded there would be major impacts on migratory ﬁsh and other protected

658 species [79]. Therefore, if the UK government were to approve such a scheme,

659 it would be vulnerable to a legal challenge under the MSFD. Any lagoon in

660 the Severn would have to consider the same receptor species and habitats

661 as the barrage, and may have to provide compensatory habitat, increasing

662 the capital cost of the project. As an example of environmental concerns

28 663 limiting the resource capture of a project, even though the Swansea Bay la-

664 goon has gained (partial) planning consent, the shape is deliberately placed

665 to minimise interference with the Tawe and Neath rivers [80].

666 The coastal zone provides humans with extensive ecosystems services, and

667 include visual amenity, including coastal seascapes [81]. Swansea Bay lagoon

668 is an example of siting a structure to mitigate visual impacts; the structure is

669 located in the northern part of Swansea Bay, next to the dock infrastructure,

670 and away from the desirable residential areas and tourist seafront located to

671 the west of the bay [80].

672 Many European countries are developing Marine Spatial Plans [82], so

673 that they have a strategic long term oversight of economic activity in the

674 oceans. The shipping industry has an historic presumption of safe navigation

675 to port, and most coastal waters have navigational zones and marked shipping

676 channels. The large scale development of lagoons could interact with these

677 channels, and any perceived impediment to navigation would be contested

678 robustly. A Marine Spatial Plan attempts to resolve these diﬀerences at

679 an early stage; however, the consequences are that lagoon shapes and sizes

680 will evolve from the most economically desirable geometry due to harbour

681 access. When other uses of the sea are taken into account, including marine

682 aggregates, oﬀshore wind, and aquaculture, the space available for lagoons

683 could be signiﬁcantly constrained. One solution could be the Multiple Use

684 of Space (MUS), with the inside of the lagoon providing an area that is

685 protected from wave action and consequently suitable for a number of other

686 uses. The MarIBE project [83] considered a number of MUS projects, and

687 proposed suitable business models for future exploitation. In particular, the

29 688 combination of aquaculture and a lagoon was investigated [84].

689 A previous project [85] considered a number of factors related to deploy-

690 ment of tidal stream turbines in the Severn Estuary, including a preliminary

691 navigational risk assessment. Although the study is not directly applicable

692 to lagoon deployment, there were two key ﬁndings. Firstly, early engagement

693 with local pilots established that the “best” location for turbines from a re-

694 source perspective was co-incident with an area of sea that is key to vessel

5 695 logistics. Secondly, the majority of the channel is 20−30 m relative to LAT ,

696 and larger container vessels are routinely 16 m draft, making large areas of

697 the channel practically unusable for the largest vessels. Applying this result

698 to all areas with high tidal range, the application of good spatial planning

699 could lead to the deeper channels available for vessels, and shallower areas

700 designated for lagoon technology.

701 Building a lagoon is a signiﬁcant item of infrastructure, and good port fa-

702 cilities are essential, in a similar way to the investments in round 3 wind farm

703 construction on the east coast of the UK [86]. Tidal Lagoon Power Plc com-

704 missioned a supply chain study that outlines the infrastructure requirements

705 [87]. Locations with theoretical resource but devoid of suitable ports in close

706 proximity may not be practical for this reason. The construction techniques

707 used also have a relevance to the port facilities required. La Rance barrage

708 made use of a Bund construction [88], and hence was eﬀectively a conven-

709 tional land based civil engineering construction. However, such methods take

710 a considerable amount of time, and may not be suitable for larger lagoons.

5Lowest Astronomical Tide.

30 711 Therefore, concrete caissons have been under consideration for a considerable

712 period of time. Clare [89] considered the caisson requirements for the 1980s

713 STPG Severn Barrage, which proposed the use of the majority of deep water

714 ports in the UK, together with towing large caissons over considerable dis-

715 tances. Finally, and importantly, a lagoon must of course be able to export

716 power to the grid, and so proximity to a suitable grid connection is a key

717 constraint.

718 5. Optimization

719 There are two main categories of tidal lagoon optimization. The ﬁrst

720 is optimization of the operation of the turbines and sluices to maximize

721 the energy yield from the lagoon, and the second is optimizing the overall

722 economic design of the lagoon to minimize the cost of energy. The academic

723 literature has focused on energy optimization, while industry tends to focus

724 more on the economics.

725 5.1. Energy optimization

726 The optimization of lagoon operation has generally been achieved through

727 the application of 0D models (Section 3.1), although other approaches have

728 been attempted. Prandle [8] used an analytical approach to solve the 0D

729 model through a number of simpliﬁcations. These included the use of a

730 single tidal constituent, a constant lagoon bathymetry, and a constant turbine

731 discharge rate.

732 Numerical solution of the 0D model has been undertaken numerous times

733 [8, 10, 26, 27, 34, 39], and is the basis for most energy yield estimates. The

734 codes seek to ﬁnd the optimal generation start and stop times, and in most

31 735 cases this is achieved through the use of ﬁxed start head values for the ebb

736 and ﬂood tides. By considering a wide range of start head values, the optimal

737 energy yield can be obtained, as shown in Fig. 4. This example plot was

738 obtained through solving the 0D conservation of mass equation using a 4th

739 order Runge-Kutta variable time-step method. Realistic turbine operation

740 paths, lagoon bathymetry and tides were used for illustrative purposes only;

741 however, the code has been applied to a range of commercial tidal energy

742 projects including the Mersey Tidal Power project and Swansea Bay Lagoon.

743 Fig. 4 clearly shows the optimal start heads for the ebb and ﬂood phases at

744 around 3.7 m and 2.7 m, respectively.

745 Yates et al. [28] have shown that energy yields can be increased through

746 the use of pumping, and this tends to be in the region of about 10% of the

747 potential energy. Due to the increase in computational power, the approach

748 typically used in industry has moved away from ﬁxed start heads to full

749 optimization of the operation path. In this approach, the basin water level is

750 discretised, and every possible path from the initial water level is calculated

751 through the required period, typically one year. The optimal path can then

752 be identiﬁed.

753 This approach is computationally expensive, and while the ﬁxed start

754 head simulations can be run in several seconds, the full optimization simu-

755 lations can take signiﬁcantly longer, with the exact time dependent on the

756 water level discretization and selected time-step. There has been very little

757 published on this approach [90], but the selection of these values is highly

758 signiﬁcant in terms of energy yield estimates. More work is needed in this

759 area.

32 760 Prandle [91] and Rainey [44] used an electrical circuit analogy to model

761 the potential energy yield of a tidal power plant. Although this approach

762 takes into account some of the potential hydrodynamic eﬀects, it does not

763 allow for the discrete operation of the lagoon, as in the standard numerical

764 approaches.

765 2D modelling tends to produce lower energy returns than 0D modelling

766 due to the impact of hydrodynamics on the system (e.g. see Section 5.3).

767 As the computational cost involved in running these models is high, few

768 optimization studies have been performed, and they tend to be used only to

769 provide an estimated correction to the 0D energy yield numbers.

770 5.2. Economic optimization

771 Economic optimization is an essential step for any realistic tidal lagoon

772 development. The operational optimization is part of this process, but a

773 much wider range of data regarding economics and other constraints (e.g.

774 environmental or practical) have to be accounted for. The basic approach is

775 to determine the Levelised Cost of Energy (LCoE) for a given lagoon design,

776 and to then vary the design to determine the minimum value [92]. The LCoE

777 is derived through:

OM C + PN n I n=1 (1 + r)n LCoE = (8) PN En n=1 (1 + r)n

778 where CI is the capital investment, OMn represents the operation and main-

779 tenance costs in year n, En is the energy yield in year n, and r is the discount

780 rate. The design of the lagoon includes the cost of the embankment, which

33 781 determines the enclosed basin area, the number and size of turbines and

782 sluices. Each design aﬀects the cost and energy yield. The optimal design

783 is found through varying all of these parameters, and yields the optimal tur-

784 bine design, number of turbines and sluices, and the optimal lagoon operation

785 path. The size and power rating of a turbine can have signiﬁcant impacts on

786 the cost of energy for a scheme, and so should be thoroughly investigated. In

787 Fig. 5, the minimum LCoE has been calculated using Eq. 8 for a ﬁxed wall

788 position for diﬀerent turbine designs. For each turbine design, the optimal

789 number of turbines and sluice gates is determined, together with the optimal

790 operating heads. The capital costs for each design are calculated through

791 simple design assumptions, and the O&M costs are ﬁxed percentages of the

792 capital. Fig. 5 shows that the optimal design, for this illustrative lagoon, is

793 a 6 m diameter 5 MW turbine. The exact number of sluices and turbines

794 and the operating heads for this turbine can then be extracted from the

795 calculated data.

796 5.3. Implications of regional hydrodynamics for individual lagoon resource

797 Lagoons act as obstructions to the otherwise undisturbed tidal dynamics

798 and will, therefore, alter natural ﬂow conditions. Accurately quantifying

799 their local and far-ﬁeld impact is crucial for ensuring their feasibility. Hydro-

800 environmental impact assessments of tidal range structures have been the

801 subject of several studies [6, 9, 24, 29, 36, 52], and it is now well established

802 that tidal impoundments can lead to changes in regional hydrodynamics,

803 with implications for existing water quality and sedimentary processes. By

804 extension, it must also be acknowledged that the presence of the lagoon may

805 impact regional tidal amplitudes and water levels.

34 806 The output of a tidal power plant is fundamentally proportional to the

807 downstream amplitude and the water head diﬀerences across the upstream

808 and downstream sides of the lagoon. Therefore, since the marine structures

809 themselves can sometimes interfere with these parameters, coastal modelling

810 tools (2D/3D) can be employed to account for the altered hydrodynamics on

811 the lagoon energy outputs. In contrast, generic 0D models assume no inter-

812 ference of the lagoon structure on regional hydrodynamics and are therefore

813 unsuitable for capturing potential losses, thereby making the expansion to

814 coupled hydrodynamic-operation models essential for accurate resource as-

815 sessment of advanced proposals. Previous studies demonstrate the disparity

816 between 0D and 2D predictions [24, 28, 52], with some indicative results

817 shown in Table 4. The general trend has been that as the project scale

818 increases, so does the hydrodynamic impact, as seen when comparing the

819 Severn Barrage and the two coastally attached tidal lagoons. However, this

820 is not an absolute; the Clwyd impoundment in the study is substantially

821 larger than the Swansea Bay lagoon, but features a lesser relative hydrody-

822 namic impact on its energy output. More factors also come into play, such

823 as the operational sequence (e.g. ebb-only, ﬂood-only or two-way) as shown

824 by the Severn Barrage STPG simulations of the particular study.

825 5.4. Multiple lagoon resource optimization

826 The tidal range structures listed in Table 4 were assessed as discrete

827 projects, but the manner that power is generated over time (Fig. 6) illustrates

828 the advantage of concurrently developing multiple tidal energy schemes. For

829 example, tidal lagoons can be strategically developed in locations that have

830 complementary tidal phases, similar to the phasing that has been suggested

35 831 for tidal stream projects [93]. For instance, projects in North Wales could

832 partially oﬀset the variability of power output from projects developed in

833 the Bristol Channel, and vice versa. However, providing continuous tidal

834 range power to the system remains a challenge during neap tides. For more

835 information, the interested reader is directed to the work of Yates et al. [28],

836 where the complementary nature of multiple tidal energy technologies has

837 been examined for the UK.

838 Introducing multiple tidal range schemes within a regional tidal system,

839 as expected, corresponds to cumulative hydrodynamic impacts, which could

840 aﬀect the energy output performance of the individual lagoons. This becomes

841 particularly pronounced once tidal power plants are developed in the same

842 channel or estuary, as with some proposals that are under consideration

843 within the Severn Estuary and Bristol Channel. It has been reported that if

844 the Swansea Bay Lagoon (Table 4) is operated in conjunction with the larger

845 Cardiﬀ Lagoon in the Severn Estuary under the same two-way operation, its

846 annual energy output is expected to reduce by approximately 2% [24]. The

847 performance of multiple lagoons could be improved through the development

848 of optimization tools that treat the operation of the plants as a system that

849 has the ﬂexibility to adapt to the transient national demand for electricity.

850 A potential advantage of having multiple small-scale projects rather than a

851 single large-scale project is that tidal power will be fed to the grid at several

852 locations rather than being concentrated at one particular point; this will

853 contribute to a more eﬃcient electricity distribution [28], and could perhaps

854 alleviate cumulative hydrodynamic impacts [24].

36 855 6. Challenges and opportunities

856 6.1. Variability and storage

857 Present UK electricity generation strategies rely on thermal power sta-

858 tions to supply the majority of baseload capacity [94]. Despatchable gener-

859 ation (e.g. gas and hydroelectric) resolves intermittency and ﬂuctuations in

860 demand [95]. The future vision is that renewable power stations will play an

861 increasing role in the generation mix, as reliance on polluting and ﬁnite fos-

862 sil fuel reserves (in addition to environmental issues associated with nuclear

863 power) is unsustainable. Although the design of 100% renewable energy sys-

864 tems is a long term goal [e.g. 96, 97], established renewable energy technolo-

865 gies such as wind and solar have issues, such as their stochastic/intermittent

866 nature, or are provided from micro generation plants distributed over large

867 geographic regions. The number one key challenge in integrating a number

868 of intermittent/variable sources into an electricity supply grid is storage [98].

869 A future strategy could involve initially implementing renewable installa-

870 tions that are complementary in phase to one another (Section 5.4), in order

871 to optimize baseload capacity and generation from these multiple sources.

872 Future steps could be to then deal with the more complex issue of load fol-

873 lowing supply and demand using supergrids or smartgrids. In-depth reviews

874 covering the potential cost and technical implications of such a task have

875 been provided by Macilwain [99], Hammons [100], and Blarke and Jenkins

876 [101].

877 Marine renewable energy, and lagoon (tidal range) power generation in

878 particular, could oﬀer the closest thing to despatchable, load-following gener-

879 ation, of any of the renewable energy sources. Scope exists to alter generation

37 880 by holding water within the impoundment for a limited period, and by pump-

881 ing into or out of the system. This is constrained by the need to allow the

882 basin to empty or ﬁll for the next cycle, and by the costs associated with

883 pumping, e.g. pumping during periods of low demand (when the cost of elec-

884 tricity is low) and recouping the costs by generating during periods of high

885 demand [10, 102], as well as the potential environmental impacts associated

886 with such an operation. The potential of tidal range power plants for storage

887 is a particularly powerful concept when we consider several plants operating

888 in harmony. Although no research has yet been conducted on this topic,

889 there is scope for optimizing the scheduling (both generating and pumping)

890 of several tidal range schemes to resolve some of the issues associated with

891 temporal variability.

892 Similarly to tidal elevations, tidal streams are also predictable, and so

893 complementary phasing of suﬃciently large tidal stream arrays, in conjunc-

894 tion with tidal lagoons, oﬀers the potential to increase baseload generation

895 capacity from multiple facets of a single renewable resource. A limitation is

896 that both tidal range and tidal streams concurrently exhibit intermittency at

897 spring/neap timescales, and so do not necessarily oﬀer peak generation dur-

898 ing times (day, week, season) of peak demand. Phase optimizing tidal energy

899 in conjunction with wind and wave energy that naturally peaks during winter

900 months [103], might help address this seasonal variability in demand; how-

901 ever, suitable predictive, coupled modelling techniques should be employed

902 to robustly assess the true generating potential and interactions between

903 technologies and schemes [e.g. 27, 104].

38 904 6.2. Additional socio-economic beneﬁts through multiple use of space

905 Tidal lagoons could be incorrectly perceived as taking up large areas of

906 sea space for very little local beneﬁt. The production of renewable electricity

907 is generally agreed to be worthwhile, but it is conceptually very diﬃcult to

908 equate one individual household’s requirements with the generating capacity

909 of a particular power station. However, a managed area of sea, protected

910 from waves by a breakwater, has signiﬁcant opportunities from Multiple Use

911 of Space (MUS) [83]. A study of MUS for the proposed Swansea Bay tidal

912 lagoon location [84] reviewed existing plans and proposed the following busi-

913 ness propositions, in addition to electricity production:

914 1. Nine million UK and one millon overseas tourists take an overnight trip

915 to Wales each year. Therefore, a visitor centre located on the lagoon

916 wall is expected to attract similar numbers per year as the existing

917 barrages in Brittany (70,000) and Nova Scotia (40,000) [21]. A boating

918 centre will be built, arts, cultural and sporting events will take place,

919 and the structure will provide amenity value for recreational ﬁshing,

920 walking and cycling.

2 921 2. Aquaculture could be developed to use some of the 11.5 km of enclosed

922 area. To improve water quality, it is proposed that Integrated Multi-

923 Trophic Aquaculture (IMTA) is implemented [105], with by-products

924 from one species feeding another. Fin ﬁsh are not recommended, as

925 these will place a high oxygen demand on the ecosystem, but a com-

926 bination of shellﬁsh and seaweed species would be suitable. These are

927 already harvested in the region. Such a concept could be extended to

928 any lagoon location, provided suitable species are selected. The market

39 929 size is expected to grow from 52.5 million tonnes in 2008 by 62% before

930 2030 [106], partly due to the depletion of wild ﬁsh stocks.

931 Overall, MUS provides sustainable, long term jobs, and fosters local owner-

932 ship of energy conversion projects, therefore helping to alleviate some of the

933 perceived negative aspects of tidal range power plants.

934 6.3. Implications of the Hendry Review

935 In February 2016 the UK Government commissioned an independent re-

936 view of tidal lagoons, entitled the Hendry Review of Tidal Lagoons, with

937 the review led by the Rt Hon Charles Hendry. Speciﬁcally, the review in-

938 vited comments on the following questions: (i) Can tidal lagoons play a

939 cost-eﬀective role as part of the UK energy mix? What is the value of the

940 energy from a UK-wide programme of lagoons? (ii) What is the potential

941 scale of opportunity in the UK? (iii) What is the potential scale of oppor-

942 tunity internationally? (iv) What are the potential structures for ﬁnancing

943 lagoons? (v) What size of lagoon should be the ﬁrst-of-a-kind (and should

944 there be one)? (vi) Could a competitive framework be put in place for the

945 delivery of tidal lagoon projects?

946 The Hendry Review was published in January 2017 [4], entitled “The

947 Role of Tidal Lagoons”, with the review supporting the development of a

948 relatively small-scale project in Swansea Bay as soon as reasonably practica-

949 ble and calling it a ‘no-regrets’ option. However, the project still requires a

950 marine licence from Natural Resources Wales, and the company promoting

951 the lagoon, namely Tidal Lagoon Power, are yet to agree a Contracts for Dif-

952 ference (CfD) price with the UK Government. A key recommendation in the

40 953 Hendry Review report was that Swansea Bay lagoon, termed a “pathﬁnder

954 project”, should be operational for a reasonable period of time before con-

955 struction commences on any larger-scale projects, so that the full range of

956 impacts can be monitored over time. This, in part, is a response to the

957 environmental, ecological and ﬁsh migration concerns raised over potential

958 lagoon impacts on marine habitats and species. Changes in the hydrody-

959 namic, water quality indicator and morphological processes can be assessed,

960 as well as the accuracy of the hydro-power predictions associated with the

961 turbines/pumps and sluice gates and their operational eﬃciencies.

962 The report makes over 30 recommendations in supporting a tidal lagoon

963 programme and delivering maximum beneﬁt to the UK, with some of the key

964 recommendations including: (i) an allocation by a competitive tender process

965 for large-scale tidal lagoons; (ii) informing the consenting process with a

966 National Policy Statement from the UK Government for tidal lagoons, similar

967 to Nuclear new build, where speciﬁc sites are designated as being suitable

968 for development; and (iii) the establishment of a new body (namely a Tidal

969 Power Authority) at arms-length from Government, with the goal being to

970 maximise the UK opportunities from a tidal lagoon programme. There is no

971 doubt that this positive and comprehensive Hendry Review towards the role

972 of tidal lagoons in the UK and internationally has raised the interest of a

973 wide range of stakeholders in developing tidal range technologies in the UK.

974 New interest and companies are now being established in a range of related

975 areas, including new turbine technologies, re-focused research programmes

976 and, in particular, increased interest from international – as well as national

977 – investors, in funding tidal range projects in the UK. Examples of projects

41 978 currently at various stages of development, in addition to the Swansea Bay

6 979 project, are Tide Mills UK & Africa which is investigating the feasibility of

980 restoring historic tide mills (and which has attracted Innovate UK funding),

981 and a much larger Severn Barrage project [107].

982 7. Conclusions

983 Following publication of the 2017 “Hendry Review”, which made over 30

984 recommendations in support of a tidal lagoon programme, tidal range power

985 plants, particularly tidal lagoons, are gaining governmental support and gen-

986 erating commercial interest. The technology that is required to build a lagoon

987 has been around for over 50 years (and has improved considerably over this

988 time period), but there are several challenges to overcome, the most pressing

989 being an assessment of the environmental impact of such schemes. However,

990 there are many opportunities, such as predictable electricity generation, and

991 the potential for tidal range power plants to provide storage.

992 This review has shown that 90% of the global tidal range resource is

993 distributed among just ﬁve countries, and that Australia is host to 30% of

994 the global tidal range resource. The review ﬁnds that concurrent strategic

995 development of multiple lagoons would minimise variability by optimizing

996 the scheduling of several such power plants operating in harmony, in addi-

997 tion to exploiting the phase diﬀerence between spatially distributed sites.

998 Finally, there is potential for cost reduction of tidal lagoon power plants by

999 considering Multiple Use of Space, for example by integrating aquaculture or

6http://gtr.ukri.org/projects?ref=132492

42 1000 combining with leisure activities.

1001 Acknowledgements

1002 This paper was the result of a two day tidal lagoon research workshop at

1003 the School of Ocean Sciences, Bangor University, 17-18 May 2016, that was

1004 funded by the Welsh Government and Higher Education Funding Council for

1005 Wales through the SˆerCymru National Research Network for Low Carbon,

1006 Energy and Environment. We thank the constructive criticisms from two

1007 anonymous reviewers on a previous version of the manuscript, which helped

1008 improve the ﬁnal, accepted, version of the paper.

1009 References

1010 [1] T. McErlean, Harnessing the Tides, The Early Medieval Tide Mills at

1011 Nendrum Monastery, Strangford Lough, Northern Ireland Archaeolog-

1012 ical Monographs No 7, TSO, 2007.

1013 [2] G. P. Hammond, C. I. Jones, R. Spevack, A technology assessment

1014 of the proposed Cardiﬀ–Weston tidal barrage, UK, in: Proceedings of

1015 the Institution of Civil Engineers-Engineering Sustainability, Thomas

1016 Telford Ltd, 2017, pp. 1–19.

1017 [3] R. H. Charlier, Forty candles for the Rance River TPP tides provide re-

1018 newable and sustainable power generation, Renewable and Sustainable

1019 Energy Reviews 11 (9) (2007) 2032–2057.

1020 [4] C. Hendry, The Role of Tidal Lagoons - Final Report (2017).

43 1021 [5] C. Baker, P. Leach, Tidal lagoon power generation scheme in Swansea

1022 Bay: A report on behalf of the Department of Trade and Industry and

1023 the Welsh Development Agency, DTI, 2006.

1024 [6] M. Kadiri, R. Ahmadian, B. Bockelmann-Evans, W. Rauen, R. Fal-

1025 coner, A review of the potential water quality impacts of tidal renew-

1026 able energy systems, Renewable and Sustainable Energy Reviews 16 (1)

1027 (2012) 329–341.

1028 [7] A. Baker, Tidal power, Proceedings of the Institution of Electrical En-

1029 gineers 134 (A5) (1987) 392–398.

1030 [8] D. Prandle, Simple theory for designing tidal power schemes, Advances

1031 in Water Resources 7 (1) (1984) 21–27.

1032 [9] A. Cornett, J. Cousineau, I. Nistor, Assessment of hydrodynamic im-

1033 pacts from tidal power lagoons in the Bay of Fundy, International Jour-

1034 nal of Marine Energy 1 (2013) 33–54.

1035 [10] G. Aggidis, D. Benzon, Operational optimisation of a tidal barrage

1036 across the Mersey estuary using 0-D modelling, Ocean Engineering 66

1037 (2013) 69–81.

1038 [11] N. Yates, I. Walkington, R. Burrows, J. Wolf, The energy gains re-

1039 alisable through pumping for tidal range energy schemes, Renewable

1040 Energy 58 (2013) 79–84.

1041 [12] G. Aggidis, Tidal range ﬂuid machinery technology and opportunities,

1042 in: Proceedings of the International Symposium on Ocean Power Fluid

1043 Machinery, London, 2010.

44 1044 [13] S. Waters, G. Aggidis, Tidal range technologies and state of the art in

1045 review, Renewable and Sustainable Energy Reviews 59 (2016) 514–529.

1046 [14] F. O. Rourke, F. Boyle, A. Reynolds, Tidal energy update 2009, Ap-

1047 plied Energy 87 (2) (2010) 398–409.

1048 [15] H. Andre, Operating experience with bulb units at the Rance tidal

1049 power plant and other French hydro-power sites, IEEE Transactions

1050 on Power Apparatus and Systems 95 (4) (1976) 1038–1044.

1051 [16] C. Retiere, Tidal power and the aquatic environment of La Rance,

1052 Biological Journal of the Linnean Society 51 (1-2) (1994) 25–36.

1053 [17] M.-C. Chaineux, R. H. Charlier, Women’s tidal power plant Forty can-

1054 dles for Kislaya Guba TPP, Renewable and Sustainable Energy Re-

1055 views 12 (9) (2008) 2515–2524.

1056 [18] J. Wu, J. Huang, X. Han, X. Gao, F. He, M. Jiang, Z. Jiang, R. B.

1057 Primack, Z. Shen, The three gorges dam: an ecological perspective,

1058 Frontiers in Ecology and the Environment 2 (5) (2004) 241–248.

1059 [19] A. Etemadi, Y. Emami, O. AsefAfshar, A. Emdadi, Electricity gener-

1060 ation by the tidal barrages, Energy Procedia 12 (2011) 928–935.

1061 [20] Y. H. Bae, K. O. Kim, B. H. Choi, Lake Sihwa tidal power plant

1062 project, Ocean Engineering 37 (5) (2010) 454–463.

1063 [21] S. Waters, G. Aggidis, A world ﬁrst: Swansea Bay Tidal lagoon in

1064 review, Renewable and Sustainable Energy Reviews 56 (2016) 916–921.

45 1065 [22] DECC, Review of Tidal Lagoons. Department of Energy and Climate

1066 Change (2016).

1067 URL https://www.gov.uk/government/news/

1068 review-of-tidal-lagoons

1069 [23] C. Baker, Tidal power, Energy Policy 19 (8) (1991) 792–797.

1070 [24] A. Angeloudis, R. A. Falconer, Sensitivity of tidal lagoon and barrage

1071 hydrodynamic impacts and energy outputs to operational characteris-

1072 tics, Renewable Energy 114 (2017) 337–351.

1073 [25] T. Hooper, M. Austen, Tidal barrages in the UK: Ecological and social

1074 impacts, potential mitigation, and tools to support barrage planning,

1075 Renewable and Sustainable Energy Reviews 23 (2013) 289–298.

1076 [26] D. Prandle, Design of tidal barrage power schemes, in: Proceedings

1077 of the Institution of Civil Engineers-Maritime Engineering, Vol. 162,

1078 Thomas Telford Ltd, 2009, pp. 147–153.

1079 [27] A. Angeloudis, R. Ahmadian, R. A. Falconer, B. Bockelmann-Evans,

1080 Numerical model simulations for optimisation of tidal lagoon schemes,

1081 Applied Energy 165 (2016) 522–536.

1082 [28] N. Yates, I. Walkington, R. Burrows, J. Wolf, Appraising the ex-

1083 tractable tidal energy resource of the UK’s western coastal waters,

1084 Philosophical Transactions of the Royal Society of London A: Mathe-

1085 matical, Physical and Engineering Sciences 371 (1985) (2013) 20120181.

1086 [29] J. Xia, R. A. Falconer, B. Lin, Hydrodynamic impact of a tidal barrage

1087 in the Severn Estuary, UK, Renewable Energy 35 (7) (2010) 1455–1468.

46 1088 [30] J. Xia, R. A. Falconer, B. Lin, Impact of diﬀerent tidal renewable

1089 energy projects on the hydrodynamic processes in the Severn Estuary,

1090 UK, Ocean Modelling 32 (1) (2010) 86–104.

1091 [31] J. Xia, R. A. Falconer, B. Lin, Impact of diﬀerent operating modes for

1092 a Severn Barrage on the tidal power and ﬂood inundation in the Severn

1093 Estuary, UK, Applied Energy 87 (7) (2010) 2374–2391.

1094 [32] J. Zhou, S. Pan, R. A. Falconer, Eﬀects of open boundary location on

1095 the far-ﬁeld hydrodynamics of a Severn Barrage, Ocean Modelling 73

1096 (2014) 19–29.

1097 [33] J. Zhou, R. A. Falconer, B. Lin, Reﬁnements to the EFDC model for

1098 predicting the hydro-environmental impacts of a barrage across the

1099 Severn Estuary, Renewable Energy 62 (2014) 490–505.

1100 [34] R. Burrows, I. Walkington, N. Yates, T. Hedges, J. Wolf, J. Holt, The

1101 tidal range energy potential of the West Coast of the United Kingdom,

1102 Applied Ocean Research 31 (4) (2009) 229–238.

1103 [35] R. Burrows, N. Yates, T. Hedges, M. Li, J. Zhou, D. Chen, I. Walk-

1104 ington, J. Wolf, J. Holt, R. Proctor, Tidal energy potential in UK

1105 waters, in: Proceedings of the Institution of Civil Engineers-Maritime

1106 Engineering, Vol. 162, Thomas Telford Ltd, 2009, pp. 155–164.

1107 [36] J. Wolf, I. A. Walkington, J. Holt, R. Burrows, Environmental im-

1108 pacts of tidal power schemes, in: Proceedings of the Institution of Civil

1109 Engineers-Maritime Engineering, Vol. 162, Thomas Telford Publishing,

1110 2009, pp. 165–177.

47 1111 [37] R. A. Falconer, J. Xia, B. Lin, R. Ahmadian, The Severn barrage and

1112 other tidal energy options: Hydrodynamic and power output modeling,

1113 Science in China Series E: Technological Sciences 52 (11) (2009) 3413–

1114 3424.

1115 [38] T. A. Adcock, S. Draper, T. Nishino, Tidal power generation–a re-

1116 view of hydrodynamic modelling, Proceedings of the Institution of

1117 Mechanical Engineers, Part A: Journal of Power and Energy (2015)

1118 0957650915570349.

1119 [39] S. Petley, G. Aggidis, Swansea Bay tidal lagoon annual energy estima-

1120 tion, Ocean Engineering 111 (2016) 348–357.

1121 [40] G. Aggidis, O. Feather, Tidal range turbines and generation on the

1122 Solway Firth, Renewable Energy 43 (2012) 9–17.

1123 [41] E. Goldwag, R. Potts, Energy production, in: Developments in tidal

1124 energy. Proceedings of the third conference on tidal power, organized

1125 by the Institution of Civil Engineers, Thomas Telford Ltd, 1989, pp.

1126 75–92.

1127 [42] S. P. Neill, E. J. Litt, S. J. Couch, A. G. Davies, The impact of tidal

1128 stream turbines on large-scale sediment dynamics, Renewable Energy

1129 34 (12) (2009) 2803–2812.

1130 [43] I. S. Robinson, Tidal power from wedge-shaped estuaries: an analyt-

1131 ical model with friction, applied to the Bristol Channel, Geophysical

1132 Journal International 65 (3) (1981) 611–626.

48 1133 [44] R. C. T. Rainey, The optimum position for a tidal power barrage in

1134 the Severn estuary, Journal of Fluid Mechanics 636 (2009) 497–507.

1135 [45] S. Draper, On the optimum place to locate a tidal fence in the Severn

1136 Estuary, in: Proceedings of the 2nd Oxford Tidal Energy Workshop,

1137 Oxford, UK, 2013.

1138 [46] R. Ahmadian, R. Falconer, B. Lin, Hydro-environmental modeling of

1139 proposed Severn barrage, UK, Proceedings of the Institution of Civil

1140 Engineers - Energy 163 (3) (2010) 107–117.

1141 [47] H. Bondi, Tidal power from the Severn Estuary: report to the Secretary

1142 of State for Energy (1981).

1143 [48] M. Hashemi, M. Abedini, S. Neill, P. Malekzadeh, Tidal and surge

1144 modelling using diﬀerential quadrature: A case study in the Bristol

1145 Channel, Coastal Engineering 55 (10) (2008) 811–819.

1146 [49] A. Angeloudis, M. Piggott, S. C. Kramer, A. Avdis, D. Coles, M. Chris-

1147 tou, Comparison of 0-D, 1-D and 2-D model capabilities for tidal range

1148 energy resource assessments, in: Proceedings of the 12th European

1149 Wave and Tidal Energy Conference (EWTEC), 27 August - 1 Septem-

1150 ber 2017, Cork, Ireland, 2017.

1151 [50] A. Avdis, T. C. Jacobs, L. S. Mouradian, J. Hill, D. M. Piggott, Mesh-

1152 ing ocean domains for coastal engineering applications, in: M. Pa-

1153 padrakakis, V. Papadopoulos, G. Stefanou, V. Plevris (Eds.), The

1154 European Community on Computational Methods in Applied Sci-

49 1155 ences and Engineering (ECCOMAS) VII Congress Proceedings, Crete,

1156 Greece, June 5–10, 2016.

1157 [51] S. Bray, R. Ahmadian, R. A. Falconer, Impact of representation of

1158 hydraulic structures in modelling a Severn barrage, Computers & Geo-

1159 sciences 89 (2016) 96–106.

1160 [52] A. Angeloudis, R. A. Falconer, S. Bray, R. Ahmadian, Representation

1161 and operation of tidal energy impoundments in a coastal hydrodynamic

1162 model, Renewable Energy 99 (2016) 1103–1115.

1163 [53] S.-H. Oh, K. S. Lee, W.-M. Jeong, Three-dimensional experiment and

1164 numerical simulation of the discharge performance of sluice passageway

1165 for tidal power plant, Renewable Energy 92 (2016) 462–473.

1166 [54] P. Jeﬀcoate, P. Stansby, D. Apsley, Flow due to multiple jets down-

1167 stream of a barrage: Experiments, 3D computational ﬂuid dynamics,

1168 and depth-averaged modeling, Journal of Hydraulic Engineering 139 (7)

1169 (2013) 754–762.

1170 [55] P. Jeﬀcoate, P. Stansby, D. Apsley, Flow and bed-shear magniﬁcation

1171 downstream of a barrage with swirl generated in ducts by stators and

1172 rotors, Journal of Hydraulic Engineering (2016) 06016023.

1173 [56] L. Carrere, F. Lyard, M. Cancet, A. Guillot, FES 2014, a new tidal

1174 model on the global ocean with enhanced accuracy in shallow seas and

1175 in the Arctic region, in: EGU General Assembly Conference Abstracts,

1176 Vol. 17, 2015, p. 5481.

50 1177 [57] R. Pawlowicz, B. Beardsley, S. Lentz, Classical tidal harmonic analysis

1178 including error estimates in MATLAB using T TIDE, Computers &

1179 Geosciences 28 (8) (2002) 929–937.

1180 [58] Alaska Resources Library and Information Services. Preliminary As-

1181 sessment of Cook Inlet Tidal Power: Phase 1 Report, Volume 2. State

1182 of Alaska Oﬃce of the Governor. pp 329. (1981).

1183 [59] N. Rayner, D. E. Parker, E. Horton, C. Folland, L. Alexander, D. Row-

1184 ell, E. Kent, A. Kaplan, Global analyses of sea surface temperature, sea

1185 ice, and night marine air temperature since the late nineteenth century,

1186 Journal of Geophysical Research: Atmospheres 108 (D14).

1187 [60] G. D. Egbert, S. Y. Erofeeva, Eﬃcient inverse modeling of barotropic

1188 ocean tides, Journal of Atmospheric and Oceanic Technology 19 (2)

1189 (2002) 183–204.

1190 [61] P. E. Robins, S. P. Neill, M. J. Lewis, S. L. Ward, Characterising the

1191 spatial and temporal variability of the tidal-stream energy resource

1192 over the northwest European shelf seas, Applied Energy 147 (2015)

1193 510–522.

1194 [62] D. Cartwright, Oceanic tides, Reports on Progress in Physics 40 (6)

1195 (1977) 665.

1196 [63] G. D. Egbert, R. D. Ray, Estimates of M2 tidal energy dissipation from

1197 TOPEX/Poseidon altimeter data, Journal of Geophysical Research:

1198 Oceans 106 (C10) (2001) 22475–22502.

51 1199 [64] M. Lewis, A. Angeloudis, P. Robins, P. Evans, S. Neill, Inﬂuence of

1200 storm surge on tidal range energy, Energy 122 (2017) 25–36.

1201 [65] K. Horsburgh, C. Wilson, Tide-surge interaction and its role in the

1202 distribution of surge residuals in the North Sea, Journal of Geophysical

1203 Research: Oceans 112 (C8).

1204 [66] K. Horsburgh, J. Williams, J. Flowerdew, K. Mylne, Aspects of op-

1205 erational forecast model skill during an extreme storm surge event,

1206 Journal of Flood Risk Management 1 (4) (2008) 213–221.

1207 [67] J. Flowerdew, K. Mylne, C. Jones, H. Titley, Extending the forecast

1208 range of the UK storm surge ensemble, Quarterly Journal of the Royal

1209 Meteorological Society 139 (670) (2013) 184–197.

1210 [68] G. A. Milne, W. R. Gehrels, C. W. Hughes, M. E. Tamisiea, Identifying

1211 the causes of sea-level change, Nature Geoscience 2 (7) (2009) 471–478.

1212 [69] J. Church, P. Clark, A. Cazenave, J. Gregory, S. Jevrejeva, A. Lever-

1213 mann, M. Merriﬁeld, G. Milne, R. Nerem, P. Nunn, A. Payne, W. Pfef-

1214 fer, D. Stammer, A. Unnikrishnan, Sea Level Change., in: T. Stocker,

1215 D. Qin, G.-K. Plattner, M. Tignor, S. Allen, J. Boschung, A. Nauels,

1216 Y. Xia, V. Bex, P. Midgley (Eds.), In: Climate Change 2013: The

1217 Physical Science Basis. Contribution of Working Group I to the Fifth

1218 Assessment Report of the Intergovernmental Panel on Climate Change,

1219 Cambridge University Press, 2013.

1220 [70] M. Lewis, K. Horsburgh, P. Bates, R. Smith, Quantifying the uncer-

52 1221 tainty in future coastal ﬂood risk estimates for the UK, Journal of

1222 Coastal Research 27 (5) (2011) 870–881.

1223 [71] M. Pickering, N. Wells, K. Horsburgh, J. Green, The impact of future

1224 sea-level rise on the European Shelf tides, Continental Shelf Research

1225 35 (2012) 1–15.

1226 [72] S. L. Ward, J. A. M. Green, H. E. Pelling, Tides, sea-level rise and

1227 tidal power extraction on the European shelf, Ocean Dynamics (2012)

1228 1153–1167.

1229 [73] R. Ahmadian, A. I. Olbert, M. Hartnett, R. A. Falconer, Sea level rise

1230 in the Severn Estuary and Bristol Channel and impacts of a Severn

1231 Barrage, Computers & Geosciences 66 (2014) 94–105.

1232 [74] H. Tang, S. Kraatz, K. Qu, G. Chen, N. Aboobaker, C. Jiang, High-

1233 resolution survey of tidal energy towards power generation and inﬂu-

1234 ence of sea-level-rise: A case study at coast of New Jersey, USA, Re-

1235 newable and Sustainable Energy Reviews 32 (2014) 960–982.

1236 [75] K. Lambeck, J. Chappell, Sea level change through the last glacial

1237 cycle., Science 292 (5517) (2001) 679–686. doi:10.1126/science.1059549.

1238 [76] P. Woodworth, F. N. Teferle, R. Bingley, I. Shennan, S. Williams,

1239 Trends in UK mean sea level revisited, Geophysical Journal Interna-

1240 tional 176 (1) (2009) 19–30.

1241 [77] P. Robins, M. Lewis, S. Neill, Changes to the resource within the life-

1242 time of a lagoon. Tidal Lagoon Workshop, Marine Centre Wales, Ban-

1243 gor University, 17-18th May 2016 (2016).

53 1244 [78] Council of European Union, Directive 2008/56/EC of the European

1245 Parliament and of the Council of 17 June 2008 establishing a frame-

1246 work for community action in the ﬁeld of marine environmental policy

1247 (Marine Strategy Framework Directive) (Text with EEA relevance)

1248 (2008).

1249 URL http://data.europa.eu/eli/dir/2008/56/oj

1250 [79] DECC, Review of Tidal Lagoons. Department of Energy and Climate

1251 Change (2010).

1252 URL https://www.gov.uk/government/publications/

1253 1-severn-tidal-power-feasibility-study-conclusions-and-summary-report

1254 [80] Tidal Lagoon Swansea Bay Plc, Environmental Statement, Tech. rep.

1255 (2013).

1256 [81] Pembrokeshire Coast National Park Authority, Seascape character as-

1257 sessment, Tech. rep. (2013).

1258 [82] W. Qiu, P. J. Jones, The emerging policy landscape for marine spatial

1259 planning in Europe, Marine Policy 39 (2013) 182–190.

1260 [83] Marine Investment in the Blue Economy (MarIBE).

1261 URL http://maribe.eu

1262 [84] R. Callaway, C. F. Gr¨unewald, Strategic Report for the combination of

1263 Tidal Lagoons, Aquaculture and Tourism, EU project MarIBE, WP8.3

1264 report, Tech. rep. (2016).

1265 [85] M. Willis, I. Masters, S. Thomas, R. Gallie, J. Loman, A. Cook, R. Ah-

1266 madian, R. A. Falconer, B. Lin, G. Gao, et al., Tidal turbine deploy-

54 1267 ment in the Bristol Channel: a case study, Proceedings of the Institu-

1268 tion of Civil Engineers-Energy 163 (3) (2010) 93–105.

1269 [86] BVG Associates, Strategic review of UK east coast staging and con-

1270 struction facilities: a report by BVG Associates for the Oﬀshore Wind

1271 Industry Council (2016).

1272 [87] Tidal Lagoon Power Plc, Ours to Own: from ﬁrst mover to mass man-

1273 ufacture, Building a new British industry from our natural advantage

1274 (2016).

1275 [88] Tidal power. Proceedings of a conference, Halifax, Nova Scotia, May

1276 1970; T.J. Gray O.K. Gashus, eds. (1972).

1277 [89] R. Clare, Construction, handling and positioning of caissons, Severn

1278 Barrage, Proc. Severn Barrage Symposium, ICivE (1982).

1279 [90] S. Ryrie, An optimal control model of tidal power generation, Applied

1280 Mathematical Modelling 19 (2) (1995) 123–126.

1281 [91] D. Prandle, Modelling of tidal barrier schemes: an analysis of the open-

1282 boundary problem by reference to AC circuit theory, Estuarine and

1283 Coastal Marine Science 11 (1) (1980) 53–71.

1284 [92] Poyry, Levelised costs of power from tidal lagoons. A report to Tidal

1285 Lagoon Power Plc. (2014).

1286 [93] S. P. Neill, M. R. Hashemi, M. J. Lewis, Tidal energy leasing and tidal

1287 phasing, Renewable Energy 85 (2016) 580–587.

55 1288 [94] National Grid, Future Energy Scenarios (2016).

1289 [95] Digest of UK Energy Statistics (DUKES) (2015).

1290 [96] B. Elliston, M. Diesendorf, I. MacGill, Simulations of scenarios with

1291 100% renewable electricity in the Australian National Electricity Mar-

1292 ket, Energy Policy 45 (2012) 606–613.

1293 [97] H. Lund, B. V. Mathiesen, Energy system analysis of 100% renewable

1294 energy systems The case of Denmark in years 2030 and 2050, Energy

1295 34 (2009) 524–531.

1296 [98] J. Kaldellis, D. Zaﬁrakis, Optimum energy storage techniques for the

1297 improvement of renewable energy sources-based electricity generation

1298 economic eﬃciency, Energy 32 (2007) 2295–2305.

1299 [99] C. Macilwain, Supergrid, Nature 468 (2010) 624–625.

1300 [100] T. Hammons, Integrating renewable energy sources into European

1301 grids, International Journal of Electrical Power & Energy Systems 30

1302 (2008) 462–475.

1303 [101] M. B. Blarke, B. M. Jenkins, Supergrid or SmartGrid: Competing

1304 strategies for large-scale integration of intermittent renewables?, En-

1305 ergy Policy 58 (2013) 381–390.

1306 [102] D. J. MacKay, Enhancing electrical supply by pumped storage in tidal

1307 lagoons, Cavendish Laboratory, University of Cambridge.

1308 [103] S. P. Neill, M. R. Hashemi, Wave power variability over the northwest

1309 European shelf seas, Applied Energy 106 (2013) 31–46.

56 1310 [104] J. Wid´en,N. Carpman, V. Castellucci, D. Lingfors, J. Olauson, F. Re-

1311 mouit, M. Bergkvist, M. Grabbe, R. Waters, Variability assessment

1312 and forecasting of renewables: A review for solar, wind, wave and tidal

1313 resources, Renewable and Sustainable Energy Reviews 44 (2015) 356–

1314 375.

1315 [105] K. Barrington, T. Chopin, S. Robinson, et al., Integrated multi-trophic

1316 aquaculture (IMTA) in marine temperate waters, Integrated maricul-

1317 ture: a global review. FAO Fisheries and Aquaculture Technical Paper

1318 529 (2009) 7–46.

1319 [106] European Commission (2015). [link].

1320 URL http://ec.europa.eu/fisheries/documentation/

1321 publications/2015-aquaculture-facts\textunderscoreen.pdf

1322 [107] R. Rainey, Lightweight steel tidal power barrages with minimal envi-

1323 ronmental impact: application to the Severn Barrage, Proc. R. Soc. A

1324 474 (2209) (2018) 20170653.

1325 [108] T. J. Hammons, Tidal power, Proceedings of the IEEE 81 (3) (1993)

1326 419–433.

57 1327 FIGURES

58 Figure 1: List of possible modes of operation, and two examples of tidal range power plant operation strategies, simulated using a 0D model and shown as time series of water elevation, ﬂow rate, and power output. (a) ebb-generation is illustrated on the left, and

(b) two-way generation on the right. ηup is the upstream water elevation (m), ηdn is the 3 downstream water elevation (m), Qs is total sluice gate ﬂow (m /s), Qt is total turbine ﬂow (m3/s), and P is Power output (GW).

59 Figure 2: The global theoretical tidal range energy resource calculated as annual energy yield (kWh/m2) per model grid cell (1/16◦ × 1/16◦).

60 Figure 3: The theoretical tidal range energy resource over the northwest European shelf seas, calculated as annual energy yield (kWh/m2). Areas landward of the [blue, red, black] contour lines denote regions with water depths less than 30 m and where energy density exceeds 84, 60, and 50 kWh/m2, respectively.

61 Figure 4: Energy yield (in GWh) obtained through a ﬁxed start head 0D model as the start head values are varied.

Figure 5: LCoE contour plot showing optimal cost (in £/MWh) as turbine design varies.

62 Figure 6: (a) Elevations, (b) hydraulic structure ﬂows, and (c) power production in the transition from a spring to a neap tide for three projects of varying scale (i.e. the Swansea Bay Lagoon (11.6 km2), the Clwyd Lagoon (126 km2), and the Severn Barrage STPG (573 km2)), assuming two-way operational sequences. Notice the phase diﬀerence between the Bristol Channel schemes (Swansea Bay Lagoon & Severn Barrage) and the Irish Sea project (Clwyd Lagoon). Adapted from Angeloudis et al. [52].

63 1328 TABLES

Table 1: Characteristics of existing tidal barrage schemes.

Power Plant Year Capacity (MW) Basin area (km2) Operation mode La Rance, France 1966 240 22 Two-way with pumping Kislaya Guba, Russia 1968 1.7 2 Two-way Annapolis Royal Generating Station, Canada 1984 20 6 Ebb only Jiangxia, China 1985 3.9 2 Two-way Lake Sihwa, Korea 1994 254 30 Flood only

64 Table 2: Tidal range locations around the world that have been identiﬁed as being tech- nically feasible [adapted from 13, 21, 108]

Country Site Type Mean tidal Basin area Proposed ca- Estimated range (m) (km2) pacity (GW) annual output (TWh)

Argentina San Jose Barrage 5.9 - 6.8 20

Australia Secure Bay 1 Barrage 10.9 - - 2.4 Secure Bay 2 Barrage 10.9 - - 2.4

Canada Cobequid Barrage 12.4 240 5.34 14 Cumberland Barrage 10.9 90 1.4 3.4 Shepody Barrage 10 115 1.8 4.8

India Gulf of Kutch Barrage 5.3 170 0.9 1.7 Gulf of Cambay Barrage 6.8 1,970 7 15

South Korea Garorim Barrage 4.7 100 0.48 0.53 Cheonsu Barrage 4.5 - - 1.2

Mexico Rio Colorado Barrage 6 − 7 - - 5.4 Tiburon Barrage - - - -

UK Severn Barrage 7.0 520 8.64 17 Mersey Barrage 6.5 61 0.7 1.5 Wyre Barrage 6.0 5.8 0.047 0.09 Conwy Barrage 5.2 5.5 0.033 0.06 Swansea Lagoon - - 0.32 - Newport Lagoon - - 0.75 - Bridgewater Lagoon - - 2 - Cardiﬀ Lagoon - - 1.8 − 2.8 - Colwyn Bay Lagoon - - 1.5 - Blackpool Lagoon - - 1.0 -

US Passamquoddy Barrage 5.5 - - - Knik Arm Barrage 7.5 - 2.9 7.4 Turnagain Arm Barrage 7.5 - 6.5 16.6

Former So- Mezen Barrage 9.1 2,300 15 50.0 viet Union Tugur Barrage - - 10 27.0 Penzhinskaya Barrage 6.0 - 50 27.0 Cauba Barrage - - - -

65 Table 3: Annual potential energy per country.

Country Annual PE (TWh) Percentage of global resource

Global (disregarding Hudson Bay) 5,792 100

Canada (Hudson) (extensive sea ice) 20,110 -

Australia 1,760 30

Canada (Fundy) 1,357 23

UK 734 13

France 732 13

US (Alaska) (partial sea ice) 619 11

Brazil 298 5

South Korea 107 2

Argentina 62 1

Russia (NW) (partial sea ice) 42 <1

Russia (NE) (partial sea ice) 33 <1

India 19 <1

China 12 <1

66 Table 4: Typical Annual Energy Predictions of a number of tidal range scheme case studies of different scales (adapted from [24] for lagoons and [52] for barrages). Hydrodynamic impact in the right hand column is defined as the difference between the 2D and 0D total accumulated energy predictions over the same simulation period, expressed as a percentage. STPG = Severn Tidal Power Group; HRC = Hydro-environmental Research Centre, Cardiff University.

Case Operation Area Location 0D Pre- 2D Pre- Hydrodynamic study (km2) diction diction impact on (TWh/yr) (TWh/yr) power production (%)

Swansea Two- 11.6 Bristol 0.53 0.49 6.8 Bay way Channel Lagoon

Clwyd Two- 125 North 2.74 2.63 3.8 Im- way Wales pounde- ment

Severn Two- 573 Severn 25.01 22.05 38.9 Barrage way Estuary HRC

Severn Ebb- 573 Severn 23.03 15.77 31.5 Barrage only Estuary STPG